There are only two other emulated audio controllers in VIrtualBox: Intel HD Audio and SoundBlaster. I tried both. Now I'm stucj because both are not recognized nor the kernel neither the oss.

Maybe this help (I mean to find where is the bug): I also have an issue with sound on OpenBSD guest - downsampling. OpenBSD als uses emulated ICH AC97. Of course I know that OpenBSD doesn't use the oss by default. But maybe this phenomenon of the downsampling will help to explain why the thing doesn't work in the oss.

You can try toggling vmix0-enabled (when nothing in the guest is using the emulated card). Also, I do find it odd hdaudio isn't recognized... I think you need to run "ossdetect -v" to fix /usr/lib/oss/etc/installed_drivers and then restart OSS (soundoff && soundon), and then it will be recognized***.

*** Edit: then again we can just modprobe oss_hdaudio manaully once soundon starts osscore as the intention is just to check this...